In an Orthogonal Frequency Division Multiplexing (OFDM) synchronous D2D communication system, maximizing the spatial resource reuse efficiency is preferred. In order to achieve maximization of the spatial resource reuse efficiency, maximizing a number of terminals in communication while maintaining Signal-to-Interference Ratio (SIR) of the recipient terminals at certain levels according to the channel condition is generally necessary.
For D2D communication without assistance of a base station, a scheduling procedure for configuring links in a distributed manner. For example, in the case of Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA) adopted by the Institute of Electrical and Electronics Engineers (IEEE) 802.11 Wireless Local Area Network (WLAN) standard, Request to Send (RTS) and Clear to Send (CTS) control frames are used. In this method, the transmitter and the receiver perform handshaking by exchanging the RTS/CTS control frames for link scheduling. For example, the D2D transmission node sends an RTS frame, and the D2D reception node sends a CTS frame in reply such that the neighboring nodes receiving this message suspend transmission temporarily. Through this RTS-CTS handshaking procedure, preventing the hidden nodes from incurring interference to the transmission node of D2D link so as to secure D2D link successfully is possible.
FIG. 1A is a diagram illustrating the RTS-CTS handshaking procedure of an asynchronous system according to the related art.
Referring to FIG. 1A, a link A-B is depicted. The node A of the link A-B sends an RTS message which is receivable within an area 110 (e.g., RTS circle). Likewise, the area 120 in which the CTS message transmitted by the node B is referred to as CTS circle 120. Within the CTS circle 120, all nodes with the exception of the node B mute transmission during a predetermined backoff time. Accordingly, the number of D2D links reusable spatially is determined based on the radius of the CTS circle (CTS radius). This method is used in the current IEEE 802.11 WLAN standard. Because the handshaking for link scheduling is performed by exchanging RTS and CTS control frames transmitted at certain timings without any synchronization configuration, this is categorized into the asynchronous scheme. At this time, because each node does not know the distance between transmission and reception nodes, the RTS and CTS radiuses have to be long enough and thus may cause spatial reuse inefficiency.
FIG. 1B is a diagram illustrating an RTS-CTS handshaking procedure of a synchronous system according to the related art.
Referring to FIG. 1B, in the OFDM-based synchronous D2D communication system, measuring mutual interferences incurring between the nodes directly or indirectly using the OFDM tones allocated to the nodes is possible. In such a case, maximizing the spatial reuse efficiency is possible by setting the RTS and CTS area to appropriate sizes. FIG. 1B is directed to a FlashLinQ system among the OFDM-based synchronous D2D communication system having RTS circles 130 (e.g., RTS circle for link A-B) and 150 (e.g., RTS circle for link C-D) and CTS circles 140 (e.g., CTS circle for link A-B) and 160 (e.g., CTS circle for link C-D). As shown in FIG. 1B, the link scheduling is performed with high spatial density. The RTS circle 130 and CTS circle 140 for link A-B may generally almost match each other. Likewise, the RTS circle 150 and CTS circle 160 for the link C-D may generally almost match each other.
The detailed link scheduling procedure for this is performed as follows. First, all D2D links are assigned unique Connection Identifiers (CIDs). Each CID is allocated a single tone corresponding to the transmission node and a single tone corresponding to the reception node according to a unique number of the CID. The link uses one of the single tone corresponding to the transmission node and the single tone corresponding to the reception node that are orthogonal to each other. The transmission node and the reception node are respectively allocated one tone, a set of the transmission node tones, and a set of the reception node tones are respectively defined as Transmission (Tx) block and Reception (Rx) block. The Tx block is the set of tones which the transmission node uses to request for scheduling, and the transmission node transmits a symbol of the tone corresponding to a corresponding CID thereof. This process is referred to as Request To Send (RTS). The RTS acts in a role similar to the RTS transmission in IEEE 802.11, and whether the reception node yield the reception is determined based on the transmission of the tone. If the reception node determines to do not yield, the reception node transmits Clear To Send (CTS) using one tone allocated in the Rx block such that the transmission node estimates an SIR at external reception node using this tone. If both the transmission and reception nodes determine to not yield, the transmission node transmits data.
FIG. 2 is a diagram illustrating a Tx block and an Rx block for use in a synchronous system according to the related art.
Referring to FIG. 2, the transmission frame (other time duration) includes a Connection Scheduling duration 210, a Rate Scheduling duration 250, a Data Segment duration 260, and an ACK duration 270. The Connection Scheduling duration 210 includes Tx block RTS signals 220 and Rx block CTS signals 230. The Connection Scheduling duration 210, the Rate Scheduling duration 250, the Data Segment duration 260, and the ACK duration 270 are categorized on the time axis.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.